If one is walking along the road and comes across a bird’s nest and the mother bird is hovering over the young, it is forbidden to take the bird and the young but one must send the mother bird away and only then take the young. The reward for performing this Mitzvah is mentioned immediately in the Torah: LEMA’AN YITAV LACH VEHA’ARACHTA YAMIM, “…so that it will be good for you and you will prolong your days.”(Deut. 22,7)
The Talmud notes that in two places longevity is mentioned as the reward for performing a Mitzvah, in the Mitzvah of sending away the mother bird and the Mitzvah of honoring one’s parents. (Ked. 39b) The difficulty with this statement is that there are other places as well. Long life is also mentioned as a reward for appointing a king and in the Shema. Why didn’t Chazal mention them?
The difference, however, is that in the two instances mentioned by Chazal it also states: LEMA’AN YITAV LACH, so that it will be good for you. (See Deut. 5,16; 22,7) That means that long life in itself is not necessarily a blessing. The blessing is when longevity is accompanied by a good life. Then one enjoys the length of days.